| Marcasite FeS2(m-FeS2)is a non-magnetic semiconductor.It has been considered as an adverse impurity to deteriorate the photovoltaic performance of pyrite FeS2 due to the small band gap value given in previous studies.In recent years,a large number of theoretical and experimental results show that the marcasite should have a band gap quite similar to that of the pyrite,possess the similar stability to that of the pyrite,and have more excellent performances than pyrite,such as high absorption coefficient,excellent thermoelectric performance,good lithium and sodium ion storage properties,large diffusion coefficient,outstanding electrochemical water oxidation performance.Therefore,marcasite is a competitive candidate in many domains.In the present paper,m-FeS2 is systematically studied based on the density functional theory,including the structural,elastic and electronic properties of m-FeS2 under pressure;the structural,electronic,magnetic and optical properties of non-metal atoms doped m-FeS2;the structural,electronic and magnetic properties of 3d transition metal atoms doped m-FeS2;effects of the vacancy and antisite defects on structural,electronic,magnetic and optical properties of m-FeS2;The surface structure,work function,electronic structure and magnetism of m-FeS2.The main conclusions are summarized as follows.(1)The m-FeS2 is an indirect nonmagnetic semiconductor with a band gap of 1.17 eV.The conduction band minimum is mainly contributed by Fe-t2g states and a little by Fe-eg states while the valence band maximum is completely contributed by Fe-t2g states,indicating that marcasite is a Fe 3d?Fe 3d charge transfer semiconductor.Marcasite is not only mechanically stable but also dynamically stable within the pressure we considered(0-15GPa).Our calculation shows the brittle nature and covalent contribution in intra-atomic bonding for marcasite.The lattice constants decrease monotonically with pressure increasing.The c axis is the easiest to compress,while b axis is the hardest to compress.The pressure has no obvious effect on the atomic positions of the atoms.With the increasing of pressure,the bulk modulus,shear modulus,Young's modulus,Poisson's ratio,density,longitudinal,transverse,average elastic wave velocities,Debye temperature and minimum thermal conductivity of m-FeS2 increase smoothly.Elastic anisotropy indexes are nonzero and increasing as pressure,which means m-FeS2 is an anisotropic material,and prone to cause stress concentration and micro-cracks.The conduction bands edge shift to higher energy while the valance bands edge shift to lower energy with pressure increasing.Accordingly,make the band gap become larger with increasing of pressure.(2)The volume of non metal atom doped m-FeS2 system increases as the atomic number of the dopant.The substitutional doping of N,P or As atoms for S site in m-FeS2 are feasible in energy under the Fe-rich condition due to the negative formation energy,especially for N doping.The phonon dispersion shows that the doped systems are dynamically stable.N-or P-doped m-FeS2 system is spin-gapless semiconductor while As-doped m-FeS2 system is a half-metal,showing the promising applications in spintronics.All the three doped systems are energetically stable in spin polarized state,indicating the existence of magnetism.The total magnetic moments of the three doped systems are all nearly 1?B due to one hole introduced by dopant,the hole-mediated p-d exchange coupling mechanism play a decisive role in the formation of magnetism.The two Fe atoms,which have the same separation with respect to dopant,are responsible for the main contributions to the total magnetic moments.In addition,the absorption coefficients exhibit red shift for N-,P-,As-doped m-FeS2 systems,the absorption coefficients are enhanced in the whole energy range for P-and As-doped systems,implying the potential applications in optoelectronic devices.(3)The volume of the 3d transition metal atom doped m-FeS2 decreases as the order of V,Cr,Mn,Co doping,and then increases as the order of Ni,Cu,Zn doping.The substitutional doping of transition metal atoms in m-FeS2 is feasible in energy under the S-rich condition due to the low formation energy.The phonon dispersion shows that the doped systems are dynamically stable.All the considered transition metal atoms doped systems are energetically stable in spin polarized state except Cu and Zn doping.V,Cr,Mn,Co and Ni-doped m-FeS2 can induce the total magnetic moments of 3,2,1,1 and 2?B,respectively.The magnetic moments come from the unpaired 3d electrons of dopants,the strong hybridizations among TM-3d,host S-3p and Fe-3d orbitals are responsible for the induced local magnetic moments of the Fe and S atoms.The Cr-doped m-FeS2 is a ferromagnetic half-metal,the V-,Mn-or Ni-doped systems are magnetic semiconductor,while non-magnetic character of Cu and Zn doping are predicted.Particularly,a single Co doping shows magnetic semiconducting while double Co doping exhibits half-metallic.(4)The defect states can be observed in m-FeS2 with various point defects.The S vacancy case is a nonmagnetic semiconductor,the band gap is smaller than perfect one due to the defect states.The Fe vacancy,FeS diavacancies and S antisite substitute Fe cases are magnetic metal,because of Fe deficiency.The Fe antisite substitute S case is a magnetic semiconductor with total magnetic moments of 2?B,making it a promising ferromagnetic semiconductor in spintronics.In addition,the real part and imaginary part of dielectric function,the absorption coefficients and refractive index of perfect and defective m-FeS2 are obtained.Both static dielectric constants and static refractive index increase,the absorption coefficients show red shifts and enhancements in the infrared region for defective m-FeS2.(5)The(010)and(110)surfaces of marcasite FeS2 with different terminations,the atoms in the outermost and subouter layers of each surface show obvious displacement after relaxation.The calculation of surface energy shows that the stability of the(010)and(110)surfaces with different terminations depends on the growing environment.All considered surfaces exhibit the spin polarized metallic characteristics,and magnetism is mainly introduced by Fe atoms with reduced coordination numbers on the outer layer. |
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